1. Field of the Invention
The present invention relates to a manufacturing method for a semiconductor device having a thin film transistor, and particularly to a technique of forming a crystalline semiconductor film used for forming an active layer of the thin film transistor.
2. Description of the Related Art
As an example of a method of forming the active layer of the thin film transistor (hereinafter, referred to as TFT), there has been developed a technique of forming an amorphous semiconductor film on a substrate having an insulating surface to crystallize the film through a laser annealing method, a thermal annealing method, or the like.
The laser annealing method is known as a crystallization technique which makes it possible to apply a high energy only to the amorphous semiconductor film to crystallize the film without significantly increasing a temperature of a glass substrate. In particular, an excimer laser oscillating a light with a short wavelength equal to or less than 400 nm is a typical laser which has been used since an initial stage of development of the laser annealing method. The laser annealing method is performed as follows: a laser beam is processed into a spot or a linear shape on an irradiated surface through an optical system and the processed laser light is scanned over the irradiated surface on the substrate (an irradiation position of the laser light is moved relative to the irradiated surface).
However, in the crystalline semiconductor film formed by the laser annealing method, a plurality of crystal grains aggregate to form the film (a grain size obtained by a conventional excimer laser crystallization method is generally about 0.1 to 0.5 μm) and the crystal grains are arranged randomly in terms of positions and sizes thereof.
The TFT manufactured on the glass substrate is formed while separating the crystalline semiconductor film into island-shaped patterns in order to separate elements. Thus, it is impossible to designate the position and the size of the crystal grain to form the TFT. Therefore, a channel formation region is hardly formed using a single crystalline semiconductor film without influence of a grain boundary.
An interface of the crystal grains (grain boundary) corresponds to a region where translational symmetry of the crystal is broken up. It is known that this reduces current transportation characteristics of carriers due to an influence of a potential barrier in a recombination center or a trapping center of the carriers or in the grain boundary, which is caused by a crystal defect or the like, and causes an OFF current to increase in the TFT.
Here, a technique called a super lateral growth is known, which can provide a large grain size as compared with the grain size in the conventional excimer laser crystallization method. This technique is described in detail by James S. Im and H. J. Kim in “On the super lateral growth phenomenon observed in excimer laser-induced crystallization of thin Si films, Appl. Phys. Lett. 64(17), 25 Apr. 1996, pp. 2303-2305”.
According to the super lateral growth, the laser light is applied to form a portion where the semiconductor film is completely melted and a portion where a solid-phase semiconductor region remains, and a crystal growth is started with the solid-phase semiconductor region used as a crystal nucleus. It takes some time for the completely melted region to exhibit nucleation, so that until the nucleation is generated in the completely melted region, the crystal is grown in a horizontal direction (hereinafter, referred to as lateral direction) with respect to a film surface of the semiconductor film with the solid-phase semiconductor region used as a crystal nucleus. Thus, the crystal grain grows to such a degree as to make its length several tens of times the film thickness. For example, a lateral crystal growth occurs with a length of 1 μm to 2 μm with respect to a silicon film having a thickness of 60 nm. Hereinafter, this phenomenon is referred to as a super lateral growth.
In the case of the above super lateral growth, a relatively large crystal grain can be obtained, but an energy intensity in an energy intensity region of a laser light to realize by the super lateral growth is extremely high as compared with that used in the general excimer laser crystallization. Also, since a range of the energy intensity region is extremely narrow, from a viewpoint of position control of the crystal grain, it is impossible to control positions where the crystal grains having a large grain size are obtained. Further, regions other than the region having the crystal grains with a large grain size become microcrystalline regions where the nucleation occurs in numerous positions or amorphous regions. In the regions, crystal grains are nonuniform in size and surface roughness of the crystal is extremely large. Therefore, an irradiation condition generally used for manufacturing the semiconductor device is a condition such that a uniform grain size of about 0.1 μm to 0.5 μm is easily obtained.
In addition, as described by Robert S. Sposili and James S. Im in “Sequential lateral solidification of thin silicon films on SiO2, Appl. Phys. Lett. 69(19), 4 Nov. 1996, pp. 2864-2866”, James S. Im et al. disclose a sequential lateral solidification method (hereinafter, referred to as SLS method) which can realize the super lateral growth at an arbitrary place under an artificial control. This SLS method is a method in which an excimer laser light of pulse oscillation is applied onto a sample through a slit-like mask. In this method, the crystallization is performed such that a relative position of the sample and the laser light is shifted by a distance approximately corresponding to a crystal length (about 0.75 μm) obtained by the super lateral growth for each shot and thus, crystal is formed continuously by using the super lateral growth that is artificially controlled.
As described above, according to the SLS method, the crystal grains subjected to the super lateral growth can be formed at an arbitrary place while being artificially controlled. However, it involves the following problems.
First, as a first problem, there can be cited poor substrate processing efficiency (throughput). As described above, in the SLS method, crystallization proceeds by a distance of about 1 μm per laser light shot. Therefore, it is necessary to make a relative movement distance (feeding pitch) of a beam spot of the laser light on a sample substrate equal to or less than 1 μm. According to the condition used for the general laser crystallization using the excimer laser of pulse oscillation, the feeding pitch per laser light shot is several tens of μm or more. Needless to say, however, the crystal peculiar to the SLS method cannot be manufactured under the above condition. The SLS method employs an XeCl excimer laser of pulse oscillation whose maximum oscillation frequency is 300 Hz. This only allows a crystallized region to be formed in such a manner that crystallization proceeds by a distance of about 300 μm at maximum per second with respect to a scanning direction of the laser light. With a processing speed at the above level, when the substrate size is enlarged, for example, to 600 mm×720 mm, a large amount of processing time per substrate is required in the conventional SLS method.
The fact that a large amount of processing time per substrate is required does not lead to only a problem in terms of time or cost. In actuality, when the amorphous semiconductor film is crystallized, processing of the surface thereof is important. For example, as pretreatment, a natural oxide film is removed using diluted hydrofluoric acid or the like and then, laser irradiation is performed in some cases. In the substrate surface, the natural oxide film may be regrown in a region subjected to laser irradiation at the end as compared with a region subjected to laser irradiation at the beginning. In this case, amounts of carbon, oxygen, and nitride elements or amounts of contamination impurities such as boron may differ in the substrate surface, these elements being taken in a completed crystal. Further, this may finally cause a variation in characteristics of a transistor in the substrate surface.
As a second problem, there can be cited the optical system that tends to be complicated in the conventional SLS method. It is required to incorporate in the optical system a mask used for processing the laser light into a slit-like shape on the substrate surface. In general, a film thickness of active layer silicon used for a polycrystalline silicon thin film transistor is several tens of nm or more. When the excimer laser of pulse oscillation is employed, a laser energy density of at least 200 mJ/cm2 (as a typical example, with respect to an amorphous silicon film having a thickness of 50 nm, about 400 mJ/cm2 in the XeCl excimer laser with a pulse width of 30 nanoseconds) is required for the laser crystallization. In the SLS method, according to an optimum condition of the super lateral growth, a slightly higher energy density region than the above is necessary. It is difficult to manufacture a slit-like mask resistant to such a high laser energy density. In a case of a mask made of metal, when a pulse laser light having a high energy density is applied, the film is locally subjected to an abrupt increase or decrease of temperature. There is concerned, for example, that peeling of the film occurs or a minute pattern shape is broken through a long-term use (in a case of photolithography performing resist exposure, a hard mask material such as chromium is used, but it is used at a low energy density beyond comparison with the laser energy density required for the silicon crystallization, so that there arises no problem concerning peeling of the film, the broken minute pattern shape, or the like). As described above, the conventional SLS method involves the complicated optical system and a factor making device maintenance difficult to perform.
Further, in order to realize the super lateral growth, it is required to make a spatial beam intensity profile of the laser light steep (to eliminate as much as possible an attenuated region in a light intensity located between an irradiation region and a nonirradiation region of the laser light). In the conventional SLS method, it is conceivable that since steep temperature slope property necessary for the super lateral growth cannot be obtained only through a general optical system which is used when the excimer laser is oscillated, the slit-like mask is required to partially shield the laser light.